Coated optical fiber, optical fiber tape core using it and optical fiber unit

ABSTRACT

The present invention provides a coated optical fiber in which a silica type optical fiber is coated with n layers (n being an integer of 2 or greater) of UV-curable resin, wherein the sum of respective contraction stress indices FI defined in the n layers of UV-curable resin by the following expression: FI [N]=(Young&#39;s modulus [MPa] of the UV-curable resin layer at −40° C.)×(cross-sectional area [mm 2 ] of the UV-curable resin layer)×(effective linear expansion coefficient [10 −6 /° C.]/10 6 )×(temperature difference 190[° C.]) is 3 [N] or less. The present invention can sufficiently prevent transmission characteristics from deteriorating in a low temperature environment.

TECHNICAL FIELD

The present invention relates to a coated optical fiber, and a coatedoptical fiber ribbon and optical fiber unit using the same.

BACKGROUND ART

Coated optical fibers used in optical communications are generally madeby coating optical fibers with a resin, whereas the resin usuallyconstitutes two layers consisting of a primary coating layer as theinner layer and a secondary coating layer as the outer layer.

An example of such coated optical fibers is disclosed in Japanese PatentApplication Laid-Open No. HEI 8-248250. The coated optical fiberdisclosed in this publication uses a primary coating layer having aYoung's modulus of 1.0 to 3.0 MPa and a glass transition point of −10°C. or lower, and a secondary coating layer having a Young's modulus ofat least 400 MPa.

DISCLOSURE OF THE INVENTION

Meanwhile, coated optical fibers are used in submarine opticalcommunications as well. Therefore, they have been desired to realizefavorable transmission characteristics in a low temperature environmentof 0 to 5° C., for example, and further at a low temperature of about−40° C. in view of such versatility as applicability to landcommunication networks.

The inventors studied the conventional coated optical fiber disclosed inthe above-mentioned publication. As a result, it was found that therewere cases where the conventional coated optical fiber disclosed in theabove-mentioned publication deteriorated its transmissioncharacteristics in a low temperature environment of 0 to 5° C. and, inparticular, there were cases where it became hard to be put intopractical use for optical communications in conformity to the wavelengthdivision multiplexing (WDM) scheme in a low temperature environment.

Therefore, it is an object of the present invention to provide a coatedoptical fiber which can sufficiently prevent transmissioncharacteristics from deteriorating in a low temperature environment, anda coated optical fiber ribbon and optical fiber unit using the same.

The inventors conducted diligent studies in order to solve theabove-mentioned problem and, as a result, have found that a coatedoptical fiber formed by coating a silica type glass optical fiber with nlayers of UV-curable resin can fully prevent transmissioncharacteristics from deteriorating at a low temperature when the sum ofcontraction stress indices FI defined according to the Young's modulusat −40° C., cross-sectional area, effective linear expansioncoefficient, and the like of each layer becomes a predetermined value orlower, thereby accomplishing the present invention.

That is, the present invention provides a coated optical fiber in whicha silica type optical fiber is coated with n layers (n being an integerof 2 or greater) of UV-curable resins, wherein the sum of respectivecontraction stress indices FI defined in the n layers of UV-curableresin by the following expression:FI [N]=(Young's modulus [MPa] of the UV-curable resin layer at −40°C.)×(cross-sectional area [mm²] of the UV-curable resinlayer)×(effective linear expansion coefficient [10⁻⁶/°C.]/10⁶)×(temperature difference 190[° C.])is 3 [N] or less.

Also, the present invention provides a coated optical fiber ribboncomprising a plurality of such coated optical fibers. Further, thepresent invention provides an optical fiber unit comprising a centraltension member and a plurality of coated optical fibers disposed aboutthe central tension member, each of the plurality of coated opticalfibers being the above-mentioned coated optical fiber.

They can fully prevent transmission loss from deteriorating at a lowtemperature, since they contain a coated optical fiber which can fullyprevent transmission loss from deteriorating at a low temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an end face view showing an embodiment of the coated opticalfiber in accordance with the present invention;

FIG. 1B is an end face view showing another embodiment of the coatedoptical fiber in accordance with the present invention;

FIG. 2 is a schematic view showing an example of drawing apparatus formaking the coated optical fiber in accordance with the presentinvention;

FIG. 3 is an end face view showing an embodiment of the coated opticalfiber ribbon in accordance with the present invention;

FIG. 4 is a sectional view showing an embodiment of the optical fiberunit in accordance with the present invention;

FIG. 5 is a schematic view showing another example of drawing apparatusfor making coated optical fibers in accordance with Examples 22 and 23;

FIG. 6 is a view for explaining a positional relationship between theguide roller and swinging guide roller in FIG. 5; and

FIG. 7 is a view for explaining a positional relationship between theswinging guide roller and fixed guide roller.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, embodiments of the present invention will beexplained.

First, the coated optical fiber in accordance with the present inventionwill be explained.

The coated optical fiber of the present invention is one obtained bycoating a silica type glass optical fiber with n layers of UV-curableresin. Here, n is an integer of 2 or greater, which is usually 2 or 3.FIG. 1A shows a coated optical fiber formed by coating the outerperiphery of a silica type glass optical fiber 1 with two layers ofUV-curable resin 2, 3, i.e., a coated optical fiber 4 whose n is 2.

Though the silica type glass optical fiber used in the coated opticalfiber of the present invention may have any refractive indexdistribution such as stepped index form as long as it is a single-modeoptical fiber, it is preferably an optical fiber having such arefractive index distribution that the dispersion at a wavelength ofabout 1.55 μm becomes zero, i.e., dispersion-shifted fiber, preferablyan NZ-type chromatic-dispersion-shifted fiber(nonzero-dispersion-shifted fiber) in particular, while its effectivecore area (A_(eff)) is preferably at least 60 μm². The effective corearea (A_(eff)) is set to at least 60 μm², since noise tends to occur dueto nonlinear phenomena when the effective core area is less than 60 μm².On the other hand, the effective core area is preferably not greaterthan 130 μm². When the effective core area exceeds 130 μm², transmissioncharacteristics become so sensitive to the bending of fiber that losstends to increase.

Another example of the silica type glass optical fiber usable in thecoated optical fiber of the present invention is a negative dispersionfiber.

Preferably, in the negative dispersion fiber, the ratio (S/D) betweenthe chromatic dispersion D and dispersion slope S at a given wavelengthwithin the wavelength range of 1.52 to 1.62 μm is 0.001 to 0.004 (1/nm).When the above-mentioned negative dispersion fiber is connected to astandard single-mode optical fiber (an optical fiber whose dispersionbecomes zero in the vicinity of a wavelength of 1.3 μm), and they areused as an optical transmission line between a light source and aphotodetector, the above-mentioned negative dispersion fiber cancompensate for the dispersion and dispersion slope of the standardsingle-mode optical fiber.

Preferably, in the negative dispersion fiber, the ratio (S/D) betweenthe chromatic dispersion D and dispersion slope S at a given wavelengthwithin the wavelength range of 1.52 to 1.62 μm is 0.004 to 0.020 (1/nm).When the above-mentioned negative dispersion fiber is connected to anonzero-dispersion-shifted optical fiber, and they are used as anoptical transmission line between a light source and a photodetector,the above-mentioned negative dispersion fiber can compensate for thedispersion and dispersion slope of the nonzero-dispersion-shiftedoptical fiber.

The above-mentioned dispersion-shifted optical fiber and negativedispersion fiber can be obtained, for example, by appropriately usinggermania (GeO₂), which increases the refractive index, and fluorine,which decreases the refractive index, so as to form a suitablerefractive index distribution form.

The outer diameter of the silica type optical fiber used in the coatedoptical fiber in accordance with the present invention is usually 115 to135 μm, preferably 124 to 126 μm.

In the coated optical fiber of the present invention, the sum ofrespective contraction stress indices FI in the n layers of UV-curableresin is 3 [N] or less. Here, the contraction stress index FI is definedby the following expression:FI [N]=(Young's modulus [MPa] of the UV-curable resin layer at −40°C.)×(cross-sectional area [mm²] of the UV-curable resinlayer)×(effective linear expansion coefficient [10⁻⁶/°C.)/10⁶)×(temperature difference 190[° C.])

In the above-mentioned expression, the effective linear expansioncoefficient takes account of not only the linear contraction caused bytemperature changes, but also the hardening shrinkage upon curing. Whena UV-curable resin composition cures within a UV irradiating apparatus,the UV-curable resin layer is at a high temperature of at least 100° C.due to the radiant heat from a UV lamp and the heat of curing reactionof the UV-curable resin composition itself. Therefore, if temperaturedecreases after the optical fiber exits the UV irradiating apparatus,the UV-curable resin layer contracts in conformity to its linearexpansion coefficient. However, the hardening shrinkage also occurs withthe curing reaction. Therefore, after the coating with the UV-curableresin composition, the outer diameter immediately prior to curingshrinks not only by the linear expansion coefficient but also by thehardening shrinkage ratio in practice. In the expression of FI, thetemperature difference can be represented by 190° C. (difference between−40° C. and 150° C.). Consequently, the effective linear expansioncoefficient is specifically represented by the following expression:α_(eff)[10⁻⁶/° C.]={average linear expansion coefficient (α_(a)[10⁻⁶/°C.] from −40° C. to 150° C.}+{shrinkage ratio (linear shrinkage ratio)s′ caused by hardening shrinkage [10⁻⁶/° C.]/190×10⁶}  (1)

Here, the linear shrinkage ratio s' is represented by the followingexpression:s′={1−(1−s)^(1/3)}  (2)where s indicates the hardening shrinkage ratio. The hardening shrinkageratio s is represented by the following expression:s=(ρ_(a)−ρ_(b))/ρ_(b)  (3)where ρ_(b) is the specific gravity of the coating layer before curing,and ρ_(a) is the specific gravity of the coating layer after curing.

When the sum of respective contraction stress indices FI in n layers ofUV-curable resin exceeds 3 [N], transmission characteristics in a lowtemperature environment deteriorate.

The lower limit of the sum of contraction stress indices FI ispreferably 0.6 N.

Among n layers of UV-curable resin, the Young's modulus of the firstUV-curable resin layer in close contact with the silica type glassoptical fiber is preferably 0.7 MPa or less, more preferably 0.5 MPa orless, at 23° C. If the Young's modulus exceeds 0.7 MPa, astrain-alleviating effect may not be exhibited when the coated opticalfiber incurs a strain, whereby transmission loss tends to increase. TheYoung's modulus of the first UV-curable resin layer at 23° C. ispreferably at least 0.1 MPa, more preferably at least 0.3 MPa. If theYoung's modulus is less than 0.1 MPa, the breaking strength of the firstUV-curable resin layer may be so low that the first UV-curable resinlayer tends to break (fracture) due to the strain applied to the coatingduring the making of the coated optical fiber.

Here, the Young's modulus of a UV-curable resin layer is measured asfollows: Namely, a sheet-like UV-curable resin layer is formed from thesame material as that of the UV-curable resin layer used for the coatedoptical fiber, and then is subjected to a tension test. As aconsequence, the Young's modulus of the UV-curable resin layer ismeasured.

In the coated optical fiber of the present invention, the adhesive forcebetween the first UV-curable resin layer in n layers of UV-curable resinand the silica type glass is preferably 50 to 200 N/m, more preferably70 to 150 N/m. When the adhesive force is less than 50 N/m, the adhesiveforce may be insufficient, whereby the probability of transmission lossincreasing at a low temperature due to the first UV-curable resin layerpeeling off from the silica type glass optical fiber tends to becomegreater. Also, in this case, the first UV-curable resin layer 2 tends topeel off from the glass optical fiber 1 as the force by which the coatedoptical fiber 24 is pressed against a swinging roller, which will beexplained later, increases. On the other hand, the operation forremoving the UV-curable resin layer tends to become difficult whenconnecting coated optical fibers if the adhesive force exceeds 200 N/m.

The breaking strength of the first UV-curable resin layer 2 in n layersof UV-curable resin is preferably at least 1.8 MPa. In this case, whenthe coated optical fiber incurs a strain, voids can sufficiently beprevented from occurring due to the deconstruction within the UV-curableresin layers, whereby the provability of transmission loss increasing ata low temperature due to the occurrence of voids can fully besuppressed. The upper limit of the breaking strength is preferably 100MPa. When the breaking strength exceeds 100 MPa, the operation forremoving the coating tends to become difficult.

Preferably, in the case where n is 3, i.e., there are three layers ofUV-curable resin, the Young's modulus of the second UV-curable resinlayer 3 at 23° C. is 150 to 1000 MPa, whereas the Young's modulus of thethird UV-curable resin layer at 23° C. is greater than 1000 MPa but notmore than 1500 MPa. In this case, microbending is further improved,whereby fractures are sufficiently prevented from occurring due toexternal damages.

As shown in FIG. 1B, the coated optical fiber in accordance with thepresent invention may be one in which n layers of UV-curable resin 2, 3are coated with a coloring layer 30. The coloring layer 30 is used foridentifying the coated optical fiber 31, and is not restricted inparticular as long as it is colored. The coloring layer 30 isconstituted by a UV-curable resin with a pigment added thereto.

The UV-curable resin layers are formed by irradiating with UV rays aresin composition adapted to cure upon UV irradiation. The resincomposition contains, for example, not only polyether urethane acrylatetype resins and the like, but also diluent resins for diluting theformer resins. The resin composition may further containphotopolymerization initiators, silane coupling agents, polar monomers,monomers having a heterocycle, monomers having a multi-membered ring,and the like when necessary.

For example, a polymerizable oligomer synthesized from 2-hydroxyethylacrylate, 2,4-tolylene diisocyanate, and polypropylene glycol; apolymerizable oligomer synthesized from 2-hydroxyethyl acrylate,2,4-tolylene diisocyanate, ethylene oxide, and tetrahydrofuran; apolymerizable oligomer synthesized from 2-hydroxyethyl acrylate,2,4-tolylene diisocyanate, and polytetramethylene glycol; or the like isused as the polyurethane acrylate type resin.

The diluent monomers are not restricted in particular as long as theycan dissolve the polyether urethane acrylate type resins and the like.Examples of the diluent monomers include monofunctional diluent monomerssuch as N-vinylpyrrolidone and N-vinylcaprolactam; multifunctionaldiluent monomers such as trimethylolpropane tri(meth)acrylate andethylene glycol di(meth)acrylate; and the like. The diluent monomer maybe a mixture of a monofunctional diluent monomer and a multifunctionaldiluent monomer. Examples of the photopolymerization initiators includebenzyldimethylketal, benzoin ethylether, 4-chlorobenzophenone,3-methylacetophenone, 2,4,6-trimethylbenzoyldiphenylphosphinoxide,thioxanthone, and the like. Examples of the silane coupling agentsinclude γ-mercaptopropyltrimethoxysilane and the like. Examples of thepolar monomers include acrylamide, N-vinylpyrrolidone,acryloylmorpholine, and the like. An example of the heterocycle islactam, whereas an example of the monomers having a heterocycle isN-vinylcaprolactam. An example of the monomers having a multi-memberedring is isobornyl acrylate.

Preferably, the first layer resin composition 7 a contains an oligomerhaving a molecular weight of at least 5000, a multifunctional monomerhaving a methylene group with a carbon number of 5 to 11, and a monomerhaving the above-mentioned heterocycle and/or a monomer having theabove-mentioned multi-membered ring, wherein the multifunctional monomerhas a weight ratio of 0.02 to 0.04 with respect to the oligomer.

When the molecular weight of the oligomer is less than 5000, the Young'smodulus of the first UV-curable resin tends to become higher.Preferably, the molecular weight of the oligomer is 30,000 or less. Whenthe molecular weight exceeds 30,000, the resulting composition yieldssuch a high viscosity that it tends to become hard to handle. An exampleof the oligomer is polyether diol.

When the carbon number of methylene group in the multifunctional monomeris less than 5, the Young's modulus of the first UV-curable resin layermay be so high that it tends to become hard to reduce the microbendingloss and remove coatings. When the carbon number of the methylene groupexceeds 11, on the other hand, the breaking strength of the firstUV-curable resin layer may be so low that voids are likely to occur inthe first UV-curable resin layer. An example of the multifunctionalmonomer is nonanediol diacrylate.

When the weight ratio of the multifunctional monomer with respect to theoligomer is less than 0.02, the breaking strength of the firstUV-curable resin layer may be so low that voids are likely to occur inthe first UV-curable resin layer. When it exceeds 0.04, on the otherhand, the Young's modulus of the first UV-curable resin layer may be sohigh that it tends to become hard to reduce the microbending loss andremove coatings.

The first layer resin composition 7 a may further contain aliphaticmonomers.

Preferably, the breaking strength of the first UV-curable resin layerobtained by curing the first layer resin composition 7 a is at least 4.0MPa. When the breaking strength is less than 4.0 MPa, voids tend tooccur in the first UV-curable resin layer.

The Young's modulus and breaking strength of a UV-curable resin layerare adjusted as follows: That is, for lowering the Young's modulus andbreaking strength of the UV-curable resin layer, it will be sufficientif the molecular weight of the polyether part of the polyether urethaneacrylate type resin is increased, or if a linear monofunctional diluentmonomer having a large molecular weight is used.

For increasing the Young's modulus and breaking strength, on the otherhand, it will be sufficient if the molecular weight of the polyetherurethane acrylate type resin is reduced, or if the rigidity of itsurethane part is raised. Alternatively, it will be sufficient if themultifunctional diluent monomer is used as a diluent monomer and itscompounding amount in the resin composition is increased, or if amonomer having a high rigidity is used as a diluent monomer.

The adhesive force between the silica type glass and the firstUV-curable resin layer can be adjusted by regulating the amount ofaddition of the polar monomer or silane coupling agent used in the firstUV-curable resin layer.

The linear expansion coefficient can be adjusted as follows: That is,the expansion at a high temperature can be reduced by decreasing theurethane bonds in the polyether urethane acrylate type resin, wherebythe average linear expansion coefficient α_(a) from −40 to 150° C. canbe lowered. Also, when a polyether urethane acrylate type resincontaining a large portion of a highly rigid part (benzene ring or thelike) is used, the linear expansion coefficient can be lowered over thewhole range of −40 to 150° C.

An example of the method of making a coated optical fiber in accordancewith the present invention will now be explained.

First, the configuration of a drawing apparatus for carrying out themethod of making the coated optical fiber in accordance with the presentinvention will be explained.

FIG. 2 is a schematic view showing an example of drawing apparatus formaking the coated optical fiber in accordance with the presentinvention. As shown in FIG. 2, a drawing apparatus 5 comprises a drawingfurnace 6 and, successively arranged in the vertical direction fromthereunder, a die 7 containing the first layer resin composition, afirst UV irradiating unit 8, a die 9 containing the second layer resincomposition, a second UV-irradiating unit 10, and a bottom roller 11.The first UV irradiating unit 8 comprises a housing 13, whereas acylindrical silica glass tube 14 for passing therethrough an opticalfiber coated with the first layer resin composition is provided in thehousing 13. A UV lamp 15 is disposed outside the glass tube 14 withinthe housing 13, whereas a reflector 16 is attached to the inside of thehousing 13. As with the first UV irradiating unit 8, the second UVirradiating unit 10 comprises a housing 17, a cylindrical silica glasstube 18, a UV lamp 19, and a reflector 20. The drawing apparatus 5further comprises a winder 12, disposed near the bottom roller 11, fortaking up the coated optical fiber 4.

For making the coated optical fiber 4 in such a drawing apparatus 5, acolumnar optical fiber preform 21 based on silica type glass isinitially prepared. The optical fiber preform 21 comprises a core partto become a core of an optical fiber, and a cladding part, disposed atthe outer periphery of the core part, to become a cladding of theoptical fiber.

The optical fiber preform 21 is passed through the drawing furnace 6, soas to melt its front end, thereby yielding an optical fiber 1. Thisoptical fiber 1 is passed through the die 7 containing the first layerresin composition, where the optical fiber 1 is coated with the firstlayer resin composition. The optical fiber 1 coated with the first layerresin composition is irradiated with UV rays by the first UV irradiatingunit 8, so as to cure the first layer resin composition, thereby coatingthe optical fiber 1 with the first UV-curable resin layer.

The optical fiber coated with the first UV-curable resin layer is passedthrough the die 9 containing the second layer resin composition, wherethe second layer resin composition is applied onto the first UV-curableresin layer. The optical fiber coated with the second layer resincomposition is irradiated with UV rays by the second UV irradiating unit10, so as to cure the second layer resin composition, thereby coatingthe first UV-curable resin layer with the second UV-curable resin layer.

This yields the coated optical fiber 4, which is then taken up by thewinder 12 by way of the bottom roller 11.

Though a case with two UV-curable resin layers is explained here by wayof example, the coated optical fiber in accordance with the presentinvention may have three or more layers.

FIG. 3 is a sectional view showing an embodiment of the coated opticalfiber ribbon in accordance with the present invention. As shown in FIG.3, the coated optical fiber ribbon 40 in accordance with this embodimentis one in which a plurality of coated optical fibers 4′ each having aUV-curable resin layer coated with a coloring layer are arranged inparallel in a tape-like coating layer 41. This coated optical fiberribbon 40 can be obtained by arranging a plurality of coated opticalfibers 4′ in parallel, coating them in this state with a resincomposition adapted to cure upon UV irradiation, for example, andirradiating the composition with UV rays so as to cure the former,thereby forming the coating layer 41.

FIG. 4 is a sectional view showing an example of the optical fiber unitin accordance with the present invention. As shown in FIG. 4, theoptical fiber unit 50 in accordance with this embodiment comprises acentral tension member 51 made of steel or the like, and a plurality ofcoated optical fibers 4′ arranged about the central tension member 51,whereas the surroundings of the coated optical fibers 4′ areasuccessively coated with a first UV-curable resin layer 52 and a secondUV-curable resin layer 53. As shown in FIG. 4, a plurality of coatedoptical fibers 4′ may be provided with respective coloring layers 54 soas to be discernible from each other.

The details of the present invention will now be explained specificallywith reference to Examples, which will not restrict the presentinvention.

EXAMPLES 1 TO 8

(Making of Coated Optical Fiber)

Using the drawing apparatus 5 shown in FIG. 2, the coated optical fiber4 was prepared as follows: First, the front end of the optical fiberpreform 1 was inserted in the drawing furnace 6 heated at 1950° C., soas to be drawn upon melting, thus yielding a dispersion-shifted opticalfiber 1 having a double core type refractive index profile, an effectivecore area of 85 μm², and an outer diameter of 125 μm. Thus obtainedproduct was passed through the die 7 containing the first layer resincomposition, so as to be coated with the first layer resin composition,which was then irradiated with UV rays by the first UV irradiating unit8, so as to be cured. Thus, the first UV-curable resin layer was formedon the optical fiber 1. As the first layer resin composition, thoselisted in Table 1 were used. As the UV lamp, a metal halide lamp wasused.

TABLE 1 Average Effective linear linear expansion expansion Contractioncoefficient coefficient stress Young's α_(a) Hardening Linear α_(eff)index Resin Inner Outer modulus (150˜ shrinkage shrinkage (150˜ Totalcom- diameter diameter Cross-sectional 23° C. −40° C. −40° C.) ratioratio −40° ) FI FI Example Layer position [mm] [mm] area [mm²] [MPa][MPa] [10⁻⁶/° C.] s[−] s′[−] [10⁻⁶/° C.] [N] [N] Example 1 1 Rs2-1 0.1250.200 0.0191 0.7 60 610 0.031 0.0104 665 0.15 1.9 2 Rh2 0.200 0.2450.0157 1000 2000 200 0.055 0.0187 298 1.78 Example 2 1 Rs2-2 0.125 0.2000.0191 0.7 60 610 0.031 0.0104 665 0.15 1.9 2 Rh2 0.200 0.245 0.01571000 2000 200 0.055 0.0187 298 1.78 Example 3 1 Rs2-3 0.125 0.200 0.01910.6 59 610 0.031 0.0104 665 0.14 1.9 2 Rh2 0.200 0.245 0.0157 1000 2000200 0.055 0.0187 298 1.78 Example 4 1 Rs2-4 0.125 0.200 0.0191 0.6 58600 0.031 0.0104 665 0.14 1.9 2 Rh2 0.200 0.245 0.0157 1000 2000 2000.055 0.0187 298 1.78 Example 5 1 Rs2-5 0.125 0.200 0.0191 0.7 60 6100.031 0.0104 665 0.15 1.9 2 Rh2 0.200 0.245 0.0157 1000 2000 200 0.0550.0187 298 1.78 Example 6 1 Rs2-6 0.125 0.200 0.0191 0.7 60 610 0.0310.0104 665 0.15 1.9 2 Rh2 0.200 0.245 0.0157 1000 2000 200 0.055 0.0187298 1.78 Example 7 1 Rs2-1 0.125 0.210 0.0224 0.7 60 610 0.031 0.0104665 0.17 1.6 2 Rh2 0.210 0.245 0.0125 1000 2000 200 0.055 0.0187 2981.42 Example 8 1 Rs2-1 0.125 0.180 0.0132 0.7 60 610 0.031 0.0104 6650.10 2.6 2 Rh2 0.180 0.245 0.0217 1000 2000 200 0.055 0.0187 298 2.46

Subsequently, the optical fiber formed with the first UV-curable resinlayer was passed through the die 9 containing the second layer resincomposition, so as to be coated with the second layer resin composition,which was then irradiated with UV rays in the second UV irradiating unit10, so as to be cured. Thus, the second UV-curable resin layer wasformed on the first UV-curable resin layer, whereby the coated opticalfiber 4 was obtained. As the second resin layer composition, thoselisted in Table 1 were used. The first and second layer resincompositions were constituted as shown in Tables 2 and 3.

TABLE 2 Amount Amount Amount Amount Resin Polyether urethane acrylate(wt %) Monomer (wt %) Photoinitiator (wt %) Silane coupling agent (wt %)Rs1 polymerizable oligomer 60 N-vinylpyrrolidone 7 2,4,6-trimethyl 2γ-mercapto 1 synthesized from isobornyl acrylate 15 benzoyldiphenylpropyltrimethoxysilane 2-hydroxyethyl acrylate, nonylphenol 15phosphinoxide 2,4-tolylene diisocyanate, EO-modified* and polypropyleneglycol (4-mole-modified)acrylate Rs2-1 polymerizable oligomer 60 laurylacrylate 10 2,4,6-trimethyl 2 γ-mercapto 3 synthesized from nonylphenolEO-modified 16 benzoyldiphenyl propyltrimethoxysilane 2-hydroxyethylacrylate, (4-mole-modified)acrylate phosphinoxide 2,4-tolylenediisocyanate, N-vinylcaprolactam 5 ethylene oxide, and triethyleneglycol 4 tetrahydrofuran diacrylate Rs2-2 polymerizable oligomer 60lauryl acrylate 12 2,4,6-trimethyl 2 γ-mercapto 3 synthesized fromnonylphenol EO-modified 16 benzoyldiphenyl propyltrimethoxysilane2-hydroxyethyl acrylate, (4-mole-modified)acrylate phosphinoxide2,4-tolylene diisocyanate, N-vinylcaprolactam 5 ethylene oxide, andtriethylene glycol 2 tetrahydrofuran diacrylate Rs2-3 polymerizableoligomer 60 lauryl acrylate 12 2,4,6-trimethyl 2 γ-mercapto 3synthesized from nonylphenol EO-modified 17 benzoyldiphenylpropyltrimethoxysilane 2-hydroxyethyl acrylate,(4-mole-modified)acrylate phosphinoxide 2,4-tolylene diisocyanate,N-vinylcaprolactam 5 ethylene oxide, and triethylene glycol 1tetrahydrofuran diacrylate Rs2-4 polymerizable oligomer 60 laurylacrylate 11 2,4,6-trimethyl 2 γ-mercapto 4 synthesized from nonylphenolEO-modified 16 benzoyldiphenyl propyltrimethoxysilane 2-hydroxyethylacrylate, (4-mole-modified)acrylate phosphinoxide 2,4-tolylenediisocyanate, N-vinylcaprolactam 5 ethylene oxide, and triethyleneglycol 2 tetrahydrofuran diacrylate Rs2-5 polymerizable oligomer 60lauryl acrylate 13 2,4,6-trimethyl 2 γ-mercapto 2 synthesized fromnonylphenol EO-modified 16 benzoyldiphenyl propyltrimethoxysilane2-hydroxyethyl acrylate, (4-mole-modified)acrylate phosphinoxide2,4-tolylene diisocyanate, N-vinylcaprolactam 5 ethylene oxide, andtriethylene glycol 2 tetrahydrofuran diacrylate

TABLE 3 Silane Polyether urethane Amount Amount Amount coupling AmountResin acrylate (wt %) Monomer (wt %) Photoinitiator (wt %) agent (wt %)Rs2-6 ″ 60 lauryl acrylate 14 ″ 2 ″ 1 nonylphenol 16 EO-modified*(4-mole-modified) acrylate N-vinylcaprolactam 5 triethylene glycol 2diacrylate Rs3 ″ 70 lauryl acrylate 8 ″ 2 ″ 1 nonylphenol 10EO-modified* (4-mole-modified) acrylate N-vinylcaprolactam 5 triethyleneglycol 4 diacrylate Rh1 polymerizable oligomer 55 lauryl acrylate 13 ″ 2synthesized from N-vinylcaprolactam 15 2-hydroxyethyl acrylate, ethyleneglycol 15 2,4-tolylene diisocyanate, diacrylate and polytetramethyleneglycol Rh2 polymerizable oligomer 60 lauryl acrylate 8 ″ 2 synthesizedfrom N-vinylcaprolactam 15 2-hydroxyethyl acrylate, ethylene glycol 152,4-tolylene diisocyanate, diacrylate and polytetramethylene glycol Rh3polymerizable oligomer 65 lauryl acrylate 15 ″ 2 synthesized fromN-vinylcaprolactam 13 2-hydroxyethyl acrylate, ethylene glycol 52,4-tolylene diisocyanate, diacrylate and polytetramethylene glycol Rh4polymerizable oligomer 60 lauryl acrylate 8 ″ 2 synthesized fromN-vinylcaprolactam 10 2-hydroxyethyl acrylate, ethylene glycol 202,4-tolylene diisocyanate, diacrylate and polytetramethylene glycol EO*:Ethylene oxide denaturation

Thus obtained coated optical fiber 4 was taken up by the winder 12 at atension of 50 g by way of the bottom roller 11.

In this coated optical fiber, the cross-sectional area, Young's modulus,average linear expansion coefficient (α_(a), linear shrinkage ratio,hardening shrinkage ratio, effective linear expansion coefficient, andFI of each of the first and second UV-curable resin layers wascalculated. Also, in thus obtained coated optical fiber, thelow-temperature transmission characteristic, microbending, adhesiveforce between the first UV-curable resin layer and silica glass, andcoating removability were studied, the status of occurrence of peeling(resistance to peeling) between the optical fiber and the firstUV-curable resin layer and voids in the first UV-curable resin layer(resistance to voids) were observed after a low-temperature test, and ahigh-tension screening test was carried out. Further, the breakingstrength was measured in each of the first and second UV-curable resinlayers.

(Calculation of Young's Modulus of UV-Curable Resin Layer)

The Young's modulus of UV-curable resin layers was measured as follows:First, the resin compositions shown in Table 2 were prepared, and wereirradiated with UV rays at a dose of 100 mJ/cm² in a nitrogenatmosphere, whereby a sheet having a thickness of 100 μm was obtained.While being shielded from light, thus obtained sheet was left for 24hours at a temperature of 23° C.±2° C. with an RH of 50±5%, so as toregulate the status of sheet. Subsequently, from this sheet, a testsample in a JIS No. 2 dumbbell shape was prepared and subjected to atension test with a marked line gap of 25 mm, a chuck gap of 25 mm, anda pulling rate of 50 mm/min, so as to calculate the Young's modulus. Therest was in conformity to JIS K7127. The Young's modulus was calculatedat each of temperatures of 23° C. and −40° C. Table 1 shows the results.

In the tension test, the Young's modulus at 23° C. was measured withTENSILON/UTM-3 manufactured by TOYO MEASURING INSTRUMENTS, whereas theYoung's modulus at −40° C. was measured with STROGRAPH-T manufactured byTOYOSEIKI SEISAKUSHO, LTD.

(Measurement of Average Linear Expansion Coefficient α_(a))

For measuring the linear expansion coefficients of the first and secondUV-curable resin layers, a film-like test piece (10 μm in thickness×5 mmin width×25 mm in length) made of the same material as that of the firstUV-curable resin layer was prepared. Similarly, a film-like test piecemade of the same material as that of the second UV-curable resin layerwas prepared. In these film-like test pieces, the average linearexpansion coefficient α_(a) (10⁻⁶/° C.) was measured with TMA (ThermalMechanical Analyzer). The average linear expansion coefficient α_(a)(10⁻⁶/° C.) was calculated as an average value of linear expansioncoefficients from −40° C. to 150° C. Table 1 shows the results.

(Calculation of Hardening Shrinkage Ratio s)

During the preparation of coated optical fiber, the specific gravity ofthe first layer resin composition and that of the first UV-curable resinlayer were measured, and the hardening shrinkage ratio s was calculatedfrom the above-mentioned expression (3). Table 1 shows the results.

(Calculation of Linear Shrinkage Ratio s′)

Based on the hardening shrinkage ratio s calculated as mentioned above,the linear shrinkage ratio s′ was calculated from the above-mentionedexpression (2). Table 1 shows the results.

(Low-Temperature Transmission Characteristic)

The low-temperature transmission characteristic was evaluated asfollows: The coated optical fiber having a length of 3000 m was wound ina ring having a diameter of about 280 mm and put into a thermostatcapable of temperature programming, the water temperature was loweredfrom 25° C. to −40° C. and then returned to 25° C., which was counted as1 cycle, and heat-cycling was carried out for 10 cycles. Here, one endof the coated optical fiber was connected to an LED light source at awavelength of 1.55 μm, whereas the other end was connected to aphotodetector, and transmission losses at 25° C. and −40° C. in 10cycles were measured. Then, the change of transmission loss at −40° C.with respect to that at 25° C. was calculated. The case where the changewas greater than 0.000 dB/km was indicated by cross mark, in which itwas considered that transmission characteristics could deteriorate in alow temperature environment. The case where the change was within therange of −0.002 dB/km to 0.000 dB/km was indicated by circle mark, inwhich it was considered that transmission characteristics in a lowtemperature environment could sufficiently be prevented fromdeteriorating. The case where the change was smaller than −0.002 dB/kmwas indicated by double circle mark, in which it was considered thattransmission characteristics in a low temperature environment could moreeffectively be prevented from deteriorating. Table 4 shows the results.

TABLE 4 Loss increase upon Loss increase Adhesive Frequency of breakingBreaking Total temperature by force Resistance to Coating uponhigh-tension strength Resistance FI drop microbending (N/m) peelingremovability screening (MPa) to void Example 1 1.9 ⊚ ∘ 150 ∘ ∘ ∘ 6.0 ∘Example 2 1.9 ⊚ ∘ 150 ∘ ∘ ∘ 2.2 ∘ Example 3 1.9 ⊚ ∘ 150 ∘ ∘ ∘ 1.1 xExample 4 1.9 ⊚ ∘ 250 ∘ x ∘ 2.3 ∘ Example 5 1.9 ⊚ ∘ 50 ∘ ∘ ∘ 2.1 ∘Example 6 1.9 ⊚ ∘ 20 x ∘ ∘ 1.8 ∘ Example 7 1.6 ⊚ ∘ — — — ∘ — — Example 82.6 ∘ ∘ — — — ∘ — —

(Evaluation of Microbending)

The transmission loss at a wavelength of 1.55 μm was measured by OTDR ineach of the states where the coated optical fiber was wound by a lengthof 600 m at a tension of 100 g about a bobbin having a shell diameter ofabout 280 mm wrapped with #1000 sandpaper and where the coated opticalfiber having a length of 1000 m was wound into a ring, and thetransmission loss in the latter state was subtracted from that in theformer state, so as to determine the transmission loss increase. Thecase where the transmission loss increase was greater than 1 dB/km wasindicated by cross mark, in which the microbending was consideredunfavorable. The case where the transmission loss increase was greaterthan 0.5 dB/km but not more than 1 dB/km was indicated by circle mark,in which the microbending was considered favorable. The case where thetransmission loss increase was not greater than 0.5 dB/km was indicatedby double circle mark, in which the microbending was considered quitefavorable. Table 4 shows the results.

(Measurement of Adhesive Force)

The adhesive force between the silica glass and the first UV-curableresin layer was measured as follows: First, a silica glass sheet wasimmersed in sulfuric acid for at least 5 minutes, so as to cleanse itssurface. The first layer resin composition was applied onto thuscleansed silica glass sheet and then was cured upon irradiation with UVrays, whereby a resin sheet having a thickness of 250 μm and a width of50 mm was formed. Here, the dose of UV irradiation was 100 mJ/cm². Thusobtained resin sheet was left for 1 week in an atmosphere at 25° C. withan RH of 50%. This resin sheet was partially peeled from the silicaglass sheet and bent to an angle of 180° , and then was peeled by 50 mmat a pulling rate of 200 mm/min. The rest was in conformity to JISZ0237. Here, the adhesive force was one in which the maximum value ofthe force at the time when peeling the resin sheet from the silica glassplate was converted into the value per unit width of the resin sheet.Table 4 shows the results.

(Observation of Status of Occurrence of Peeling Between Optical Fiberand First UV-Curable Resin Layer After Low-Temperature TransmissionCharacteristic Test)

The coating state of the coated optical fiber after the low-temperaturetransmission characteristic test was verified by immersing the coatedoptical fiber in a matching oil for refractive index adjustment afterthe low-temperature transmission characteristic test and then observingthe coated optical fiber in a side face direction thereof with anoptical microscope under a magnification of 50×. Then, the case wherethe first UV-curable resin layer was peeled from the optical fiber wasindicated by cross mark, in which it was considered that the peeling waslikely to occur (there was no resistance to peeling) in the lowtemperature environment. The case where the first UV-curable resin layerwas not peeled from the optical fiber was indicated by circle mark, inwhich it was considered that the peeling was sufficiently hard to occur(there was a resistance to peeling) in the low temperature environmentas well. Table 4 shows the results.

(Evaluation of Coating Removability)

For evaluating the coating removability of coated optical fiber, acoated optical fiber ribbon was prepared. For preparing the coatedoptical fiber ribbon, four coated optical fibers each obtained asmentioned above were arranged in parallel, and coated with a UV-curableresin composition, which was then cured upon irradiation with UV rays,whereby the four coated optical fibers were collectively coated. Used asthe UV-curable resin composition was one containing 70% by weight of apolymerizable oligomer synthesized from 2-hydroxyethyl acrylate,2,4-tolylene diisocyanate, and polypropylene glycol, 28% by weight ofN-vinylpyrrolidone as a diluent monomer, and 2% by weight of2,4,6-trimethylbenzoylphenylphosphine oxide.

The removability of the collective coating in thus obtained coatedoptical fiber ribbon was evaluated as follows: Namely, the coating onone end of the coated optical fiber ribbon was manually removed at oncewith a heated remover (JR-4A; manufactured by Sumitomo ElectricIndustries, Ltd.). The heating temperature of the heater for the heatedremover was 90° C. Table 4 shows the results of coating removability atthis time. In Table 4, the case where the four glass fibers could beexposed was indicated by circle mark, in which the coating removabilityof coated optical fiber was considered favorable; whereas the case wherethe glass fibers could not be exposed was indicated by cross mark, inwhich the coating removability of coated optical fiber was consideredunfavorable.

(Measurement of Breaking Frequency upon High-Tension Screening)

Screening was carried out while applying a tensile tension of 21.6 N tothe coated optical fiber, so as to study the breaking frequency of thecoated optical fiber. The case where the breaking frequency was{fraction (5/1000)} km or less was indicated by circle mark, in whichbreaking was considered hard to occur upon high-tension screening;whereas the other case was indicated by cross mark. Table 4 shows theresults.

(Measurement of Breaking Strength)

The breaking strength of the first UV-curable resin layer was determinedas follows: The material same as that of the first layer resincomposition was applied onto a silica glass substrate, and was curedupon irradiation with UV rays at a dose of 100 mJ/cm² in a nitrogenatmosphere, whereby a sheet having a thickness of about 100 μm wasobtained. The sheet was formed into a JIS No. 2 dumbbell shape. Thestatus of thus obtained sheet was regulated for at least 24 hours at atemperature of 23° C.±2° C. with an RH of 50±5% while being shieldedfrom light. Then, using a tension tester (TENSILON/UTM-3; manufacturedby TOYO MEASURING INSTRUMENTS), the sheet was subjected to a tensiontest with a marked line gap of 25 mm, a chuck gap of 80±5 mm, and apulling rate of 50 mm/min, until it broke, and the stress at the time ofbreaking was taken as the breaking strength. The rest in the tensiontest was in conformity to JIS K7127. Table 4 shows the results.

(Observations of Status of Peeling at Interface Between Optical Fiberand First UV-Curable Resin Layer and Voids in First UV-Curable ResinLayer)

After being subjected to the high-tension screening, the coated opticalfiber was reeled out from the take-up bobbin, so as to be immersed in amatching oil for refractive index adjustment, and then a side facethereof was observed under a microscope of 50×, so as to determine thestatus of peeling and whether voids exist or not. Table 4 shows theresults. In Table 4, the case where a void or peeling was seen wasindicated by cross mark, in which it was considered that there was noresistance to void; whereas the case where neither void nor peeling wasseen was indicated by circle mark, in which it was considered that therewas a resistance to void.

EXAMPLES 9 AND 10

A coated optical fiber was made in the same manner as Example 1, 7, or8, except that the third UV-curable resin layer obtained by the resincompositions listed in Table 5 was further provided on the secondUV-curable resin layer, and that the outer diameters of the first tothird UV-curable resin layers were set to their respective values shownin Table 5.

TABLE 5 Average Effective linear linear expansion expansion Contractioncoefficient coefficient stress Young's α_(a) Hardening Linear α_(eff)index Resin Inner Outer modulus (150˜ shrinkage shrinkage (150˜ Totalcompo- diameter diameter Cross-sectional 23° C. −40° C. −40° C.) ratioratio −40° C.) FI FI Example Layer sition [mm] [mm] area [mm²] [MPa][MPa] [10⁻⁵/° C.] s[−] s′[−] [10⁻⁵/° C.] [N] [N] Example 1 Rs2-1 0.1250.200 0.0191 0.7 60 610 0.031 0.0104 665 0.15 1.9  9 2 Rh2 0.200 0.2350.0120 1000 2000 200 0.055 0.0187 298 1.36 3 Rh4 0.235 0.245 0.0038 12003000 100 0.050 0.0170 189 0.41 Example 1 Rs2-1 0.125 0.210 0.0224 0.7 60610 0.031 0.0104 665 0.17 1.6 10 2 Rh2 0.210 0.235 0.0087 1000 2000 2000.055 0.0187 298 0.99 3 Rh4 0.235 0.245 0.0038 1200 3000 100 0.0500.0170 189 0.41 Example 1 Rs1 0.125 0.200 0.0191 1200 62 580 0.0320.0108 637 0.14 1.9 11 2 Rh2 0.200 0.245 0.0157 1000 2000 200 0.0550.0187 298 1.78 Example 1 Rs1 0.125 0.210 0.0224 1200 62 580 0.0320.0108 637 0.17 1.6 12 2 Rh2 0.210 0.245 0.0125 1000 2000 200 0.0550.0187 298 1.42

TABLE 6 Average Effective linear linear expansion expansion Contractioncoefficient coefficient stress Young's α_(a) Hardening Linear α_(eff)index Resin Inner Outer modulus (150˜ shrinkage shrinkage (150˜ Totalcom- diameter diameter Cross-sectional 23° C. −40° C. −40° C.) ratioratio −40° ) FI FI Example Layer position [mm] [mm] area [mm²] [MPa][MPa] [10⁻⁵/° C.] s[−] s′[−] [10⁻⁵/° C.] [N] [N] Example 1 Rs1 0.1250.200 0.0191 1200 62 580 0.032 0.0108 637 0.14 1.9 13 2 Rh2 0.200 0.2350.0120 1000 2000 200 0.055 0.0187 298 1.36 3 Rh4 0.235 0.245 0.0038 12003000 100 0.050 0.0170 189 0.41 Example 1 Rs3 0.125 0.210 0.0224 0.3 50650 0.031 0.0104 705 0.15 0.6 14 2 Rh3 0.210 0.245 0.0125 150 600 1900.057 0.0194 292 0.42 Example 1 Rs3 0.125 0.210 0.0224 0.3 50 650 0.0310.0104 705 0.15 0.8 15 2 Rh3 0.210 0.235 0.0087 150 600 190 0.057 0.0194292 0.29 3 Rh4 0.235 0.245 0.0038 1200 3000 100 0.050 0.0170 189 0.41Example 1 Rs3 0.125 0.200 0.0191 0.3 50 650 0.031 0.0104 705 0.13 1.0 162 Rh3 0.200 0.235 0.0120 150 600 190 0.057 0.0194 292 0.40 3 Rh2 0.2350.245 0.0038 1000 2000 200 0.055 0.0187 298 0.43 Com- 1 Rs2-1 0.1250.180 0.0132 0.7 60 610 0.031 0.0104 665 0.10 3.4 parative 2 Rh1 0.1800.245 0.0217 1500 2600 205 0.060 0.0204 312 3.35 Example  1

In this coated optical fiber, the cross-sectional area, Young's modulus,average linear expansion coefficient α_(a), linear shrinkage ratio,hardening shrinkage ratio, and effective linear expansion coefficientα_(eff) in each of the first to third UV-curable resin layers werecalculated as in Example 1, 7, or 8. Then, the respective contractionstress indices FI of the individual layers, and their sum weredetermined. Table 5 shows the results.

Further, thus obtained coated optical fiber was studied in terms oflow-temperature transmission and microbending, and was subjected to ahigh-tension screening test. Table 7 shows the results.

TABLE 7 Loss increase Loss upon increase Frequency of breaking Totaltemperature by micro- upon high-tension FI drop bending screeningExample 9 1.9 ⊚ ⊚ ∘ Example 10 1.6 ⊚ ⊚ ∘ Example 11 1.9 ⊚ x ∘ Example 121.6 ⊚ x ∘ Example 13 1.9 ⊚ x ∘ Example 14 0.6 ⊚ ∘ x Example 15 0.8 ⊚ ⊚ ∘Example 16 1.0 ⊚ ∘ ∘ Comparative 3.4 x ⊚ ∘ Example

EXAMPLE 11

Coated optical fibers were made in the same manner as Examples 1 to 6except that Rs1 was used in place of Rs2-1.

In these coated optical fibers, the cross-sectional area, Young'smodulus, average linear expansion coefficient α_(a), linear shrinkageratio, hardening shrinkage ratio, and effective linear expansioncoefficient α_(eff) in each of the first and second UV-curable resinlayers were calculated as in Examples 1 to 6. Then, the respectivecontraction stress indices FI of the individual layers, and their sumwere determined. Table 5 shows the results.

Further, thus obtained coated optical fibers were studied in terms oflow-temperature transmission and microbending, and were subjected to ahigh-tension screening test. Table 7 shows the results.

EXAMPLE 12

Two kinds of coated optical fibers were made in the same manner asExample 7 except that Rs1 was used as the first layer resin composition.

In these coated optical fibers, the cross-sectional area, Young'smodulus, average linear expansion coefficient α_(a), linear shrinkageratio, hardening shrinkage ratio, and effective linear expansioncoefficient α_(eff) in each of the first and second UV-curable resinlayers were calculated as in Example 7. Then, the respective contractionstress indices FI of the individual layers, and their sum weredetermined. Table 5 shows the results.

Further, thus obtained coated optical fibers were studied in terms oflow-temperature transmission and microbending, and were subjected to ahigh-tension screening test. Table 7 shows the results.

EXAMPLE 13

A coated optical fiber was made in the same manner as Example 9 exceptthat Rs1 was used in place of Rs2-1 as the first layer resincomposition.

In this coated optical fiber, the cross-sectional area, Young's modulus,average linear expansion coefficient α_(a), linear shrinkage ratio,hardening shrinkage ratio, and effective linear expansion coefficientα_(eff) in each of the first to third UV-curable resin layers werecalculated as in Example 9. Then, the respective contraction stressindices FI of the individual layers, and their sum were determined.Table 6 shows the results.

Further, thus obtained coated optical fiber was studied in terms oflow-temperature transmission and microbending, and was subjected to ahigh-tension screening test. Table 7 shows the results.

EXAMPLE 14

A coated optical fiber was made in the same manner as Example 7 exceptthat Rs3 was used in place of Rs2-1 as the first layer resincomposition, and that Rh3 was used in place of Rh2 as the second layerresin composition.

In this coated optical fiber, the cross-sectional area, Young's modulus,average linear expansion coefficient α_(a), linear shrinkage ratio,hardening shrinkage ratio, and effective linear expansion coefficientα_(eff) in each of the first and second UV-curable resin layers werecalculated as in Example 7. Then, the respective contraction stressindices FI of the individual layers, and their sum were determined.Table 6 shows the results.

Further, thus obtained coated optical fiber was studied in terms oflow-temperature transmission and microbending, and was subjected to ahigh-tension screening test. Table 7 shows the results.

EXAMPLE 15

Coated optical fibers were made in the same manner as Example 10 exceptthat those shown in Table 6 were used as the first to third layer resincompositions.

In these coated optical fibers, the cross-sectional area, Young'smodulus, average linear expansion coefficient α_(a), linear shrinkageratio, hardening shrinkage ratio, and effective linear expansioncoefficient α_(eff) in each of the first to third UV-curable resinlayers were calculated as in Example 10. Then, the respectivecontraction stress indices FI of the individual layers, and their sumwere determined. Table 6 shows the results.

Further, thus obtained coated optical fibers were studied in terms oflow-temperature transmission and microbending, and were subjected to ahigh-tension screening test. Table 7 shows the results.

EXAMPLE 16

Coated optical fibers were made in the same manner as Examples 9 and 13except that those shown in Table 6 were used as the first to third resincompositions.

In these coated optical fibers, the cross-sectional area, Young'smodulus, average linear expansion coefficient α_(a), linear shrinkageratio, hardening shrinkage ratio, and effective linear expansioncoefficient α_(eff) in each of the first to third UV-curable resinlayers were calculated as in Examples 9 and 13. Then, the respectivecontraction stress indices FI of the individual layers, and their sumwere determined. Table 6 shows the results.

Further, thus obtained coated optical fibers were studied in terms oflow-temperature transmission and microbending, and were subjected to ahigh-tension screening test. Table 7 shows the results.

COMPARATIVE EXAMPLE 1

A coated optical fiber was made in the same manner as Example 8 exceptthat Rh1 was used in place of Rh2 as the second layer resin composition.

In this coated optical fiber, the cross-sectional area, Young's modulus,average linear expansion coefficient α_(a), linear shrinkage ratio,hardening shrinkage ratio, and effective linear expansion coefficientα_(eff) in each of the first and second UV-curable resin layers werecalculated as in Example 8. Then, the respective contraction stressindices FI of the individual layers, and their sum were determined.Table 6 shows the results.

Further, thus obtained coated optical fiber was studied in terms oflow-temperature transmission and microbending, and was subjected to ahigh-tension screening test. Table 7 shows the results.

EXAMPLES 17 TO 19

A coated optical fiber was made in the same manner as Example 1, 7, or 8except that Rs2-0 in Table 8 was used in place of Rs2-1 as the firstlayer resin composition, that Rh2-0 in Table 8 was used in place of Rh2as the second layer resin composition, and that the inner and outerdiameters of the first and second UV-curable resin layers were set totheir respective values shown in Table 9. In Table 8, the substanceslisted in the column of “urethane acrylate” indicate the materials forurethane acrylate, whereas the molar ratios indicate those of thesematerials. Further, “part” refers to the part of urethane acrylate.

TABLE 8 Resin Urethane acrylate molar composition (oligomer) ratio partMonomer part Photoinitiator part Rs2-0 polyether diol 2 65 isobornylacrylate 14 Lucirin 1.5 (molecular weight: N-vinylcaprolactam 8 TPO7000) nonylphenol acrylate 8 isophorone 3 nonanediol 2 diisocyanatehydroxyethyl 2 acrylate Rh2-0 ethylene-oxide-added 1 15 isobornylacrylate 13 Lucirin 1.5 diol of bisphenol-A isobornyl acrylate 10 TPOisophorone 2 diacrylate of 20 diisocyanate ethylene-oxide-addedhydroxyethyl 2 diol of bisphenol-A acrylate polytetramethylene 1 30glycol isophorone 2 diisocyanate hydroxyethyl 2 acrylate isophorone 1 10diisocyanate hydroxyethyl 2 acrylate Resin Silane composition couplingagent part Antioxidant part Rs2-0 γ-mercapto 13,9-bis[2-{3-(3-tert-butyl-4-hydroxy- 0.5 propyltrimeth-5-methylphenyl)propyonyloxy}-1,1- oxysilane dimethylethyl]-2,4,8,10-tetraoxaspiro[5,5]-undecane Rh2-0 —3,9-bis[2-{3-(3-tert-butyl-4-hydroxy- 0.55-methylphenyl)propyonyloxy}-1,1- dimethylethyl]-2,4,8,10-tetraoxaspiro[5,5]-undecane

TABLE 9 Average Effective linear linear expansion expansion Contractioncoefficient coefficient stress Young's α_(a) Hardening Linear α_(eff)index Resin Inner Outer modulus (150˜ shrinkage shrinkage (150˜ Totalcom- diameter diameter Cross-sectional 23° C. −40° C. −40° C.) ratioratio −40° ) FI FI Example Layer position [mm] [mm] area [mm²] [MPa][MPa] [10⁻⁵/° C.] s[−] s′[−] [10⁻⁵/° C.] [N] [N] Example 1 Rs2-0 0.1250.180 0.0132 0.7 48 600 0.036 0.0121 664 0.08 2.27 17 2 Rh2-0 0.1800.245 0.0217 882 1862 190 0.053 0.0180 285 2.19 Example 1 Rs2-0 0.1250.200 0.0191 0.7 48 600 0.036 0.0121 664 0.12 1.70 18 2 Rh2-0 0.2000.245 0.0157 882 1862 190 0.053 0.0180 285 1.58 Example 1 Rs2-0 0.1250.210 0.0224 0.7 48 600 0.036 0.0121 664 0.14 1.40 19 2 Rh2-0 0.2100.245 0.0125 882 1862 190 0.053 0.0180 285 1.26 Example 1 Rs2-0 0.1250.200 0.0191 0.7 48 600 0.036 0.0121 664 0.12 1.73 20 2 Rh2-0 0.2000.235 0.0120 882 1862 190 0.053 0.0180 285 1.20 3 Rh4 0.235 0.245 0.00381200 3000 100 0.050 0.0170 189 0.41 Example 1 Rs2-0 0.125 0.210 0.02240.7 48 600 0.036 0.0121 664 0.14 1.43 21 2 Rh2-0 0.210 0.235 0.0087 8821862 190 0.053 0.0180 285 0.88 3 Rh4 0.235 0.245 0.0038 1200 3000 1000.050 0.0170 189 0.41

TABLE 10 Frequency of breaking Loss increase Loss increase by Adhesiveforce Coating upon high-tension Resistance to Total FI upon temperaturedrop microbending (N/m) removability screening void Example 2.27 ⊚ ∘ 100∘ ∘ ∘ 17 Example 1.70 ⊚ ∘ 100 ∘ ∘ ∘ 18 Example 1.40 ⊚ ∘ 100 ∘ ∘ ∘ 19Example 1.73 ⊚ ⊚ — — ∘ — 20 Example 1.43 ⊚ ⊚ — — ∘ — 21

In this coated optical fibers, the cross-sectional area, Young'smodulus, average linear expansion coefficient α_(a), linear shrinkageratio, hardening shrinkage ratio, and effective linear expansioncoefficient α_(eff) in each of the first and second UV-curable resinlayers were calculated as in Example 1, 7, or 8. Then, the respectivecontraction stress indices FI of the individual layers, and their sumwere determined. Table 9 shows the results.

Further, thus obtained coated optical fiber was studied in terms oflow-temperature transmission and microbending characteristics, and wassubjected to a high-tension screening test. Table 10 shows the results.

EXAMPLES 20 AND 21

A coated optical fiber 4 was made in the same manner as Examples 17 to19 except that the third TV-curable resin layer obtained from the resincomposition Rh4 in Table 3 was further provided on the second UV-curableresin layer, and that the inner and outer diameters of the first tothird UV-curable resin layers were set to their respective values shownin Table 7.

In this coated optical fiber 4, the cross-sectional area, Young'smodulus, average linear expansion coefficient α_(a), linear shrinkageratio, hardening shrinkage ratio, and effective linear expansioncoefficient α_(eff) in each of the first to third UV-curable resinlayers were calculated as in Examples 17 to 19. Then, the respectivecontraction stress indices FI of the individual layers, and their sumwere determined. Table 9 shows the results.

Further, as in Examples 17 to 19, thus obtained coated optical fiber 4was studied in terms of low-temperature transmission and microbending,and was subjected to a high-tension screening test. Table 10 shows theresults.

EXAMPLE 22

A coated optical fiber was made in the following manner by amanufacturing apparatus equipped with a swinging guide roller shown inFIG. 5.

FIG. 5 is a schematic view showing the apparatus for making a coatedoptical fiber used in this example, illustrating the process until theglass optical fiber drawn from an optical fiber preform is taken up by awinder. In FIG. 5, 22 refers to a laser outer diameter meter, 23 adrawing controller, 7 a a first layer resin composition, 9 a a secondlayer resin composition, 24 a coated optical fiber, 25 a guide roller,26 a swinging guide roller, and 27, 28 fixed guide rollers. Here,constituents identical or equivalent to those in FIG. 2 were referred towith numerals identical thereto.

An optical fiber preform 21 was set in the drawing furnace 6, so thatthe front end of the optical fiber preform 21 was inserted in thedrawing furnace 6 heated at 1950° C. by the heater 6 a and drawn uponmelting, whereby a glass optical fiber 1 was obtained. The glass opticalfiber 1 was made as a dispersion-shifted fiber having a double core typerefractive index profile, an effective area of 85 μm², and an outerdiameter of 125 μm. The drawing rate was 100 m/min.

The outer diameter of the drawn glass optical fiber 1 was measured bythe laser outer diameter meter 22. The result of measurement of theouter diameter of glass optical fiber 1 was fed back to the drawingcontroller 23, and the heating temperature of the heater 6 a and thedrawing rate of the glass optical fiber 1 were regulated so as to attaina desirable outer diameter.

Then, the glass optical fiber 1 drawn to a predetermined outer diameterwas passed through the die 7 containing the first layer resincomposition 7 a, so as to be coated with the first layer resincomposition 7 a, which was then cured upon irradiation with UV rays inthe first UV irradiating unit 8. Thus, the first UV-curable resin layerwas formed on the optical fiber 1.

Subsequently, the optical fiber formed with the first UV-curable resinlayer was passed through the die 9 containing the second layer resincomposition 9 a, so as to be coated with the second layer resincomposition 9 a, which was then cured upon irradiation with UV rays inthe second UV irradiating unit 10. Thus, two layers of UV-curable resinwere formed on the glass optical fiber 1, whereby the coated opticalfiber 24 was obtained.

Used as the first layer resin composition was one in which the amount ofmultifunctional monomer (nonanediol diacrylate) was adjusted in thecomposition of Rs2-0 shown in Table 8, whereas Rh2-0 shown in Table 8was used as the second layer resin composition. As the UV lamps in theUV irradiating units 8, 10, metal halide lamps were used.

Thus obtained coated optical fiber 24 was taken up by the winder 12 byway of the guide roller 25, swinging guide roller 26, and fixed guiderollers 27, 28.

When passing through the guide roller 25, the coated optical fiber 24was guided through a gap (about 2 mm) between a roller pair 25 a, and agap (about 2 mm) between a roller pair 25 b as shown in FIG. 6.

Also, the coated optical fiber 24 was alternately twisted along itsmoving direction as follows:

Namely, as shown in FIG. 7, the rotary axis y of the swinging guideroller 26 was pivoted to +θ about the drawing direction axis z, so as toimpart a lateral force to the coated optical fiber 24, thereby rotatingthe coated optical fiber 24 on the surface of the swinging guide roller26, which provided the coated optical fiber 24 with a twist.Subsequently, the swinging guide roller 26 was pivoted in the oppositedirection to −θ, so that the coated optical fiber 24 rotated on thesurface of the swinging guide roller 26 in the opposite direction. Thus,the swinging guide roller 26 was repeatedly pivoted from +θ to −θ,whereby the coated optical fiber 24 was alternately provided withclockwise and counterclockwise twists with respect to the movingdirection thereof. In this case, the drawing tension T of the glassoptical fiber 1 was 2.5 (N/fiber), the radius R of swinging guide rollerwas 0.08 (m), T/R was 31.3, and the number of pivotal swings was 1.67(s⁻¹), each being made constant. Here, the number of pivotal swings wasindicated by the number of pivotal movements of the swinging guideroller per second, one pivotal movement being one cycle from +θ to −θand then from −θ to +θ.

Reference number 29 is a V-shaped groove for preventing the coatedoptical fiber 24 from rotating on the surface of the roller 27.

In thus obtained coated optical fiber 24, the cross-sectional area,Young's modulus, average linear expansion coefficient α_(a), linearshrinkage ratio, hardening shrinkage ratio, effective linear expansioncoefficient, and FI in each of the first and second UV-curable resinlayers were calculated. The results were the same as those in Example17.

Also, as in Examples 1 to 8, thus obtained coated optical fiber 24 wasstudied in terms of low-temperature transmission and microbending, andwas subjected to a high-tension screening test. Their results were thesame as those in Example 17. Further, the breaking strength of the firstUV-curable resin layer was measured and found to be 4.0 MPa.

Furthermore, thus obtained optical fiber 24 was evaluated in terms ofthe occurrence of voids and the peeling between the glass optical fiberand the first UV-curable resin layer at their interface. Table 11 showsthe results. In Table 11, the criteria for determining the occurrencesof voids and peeling were the same as those in Examples 1 to 8.

TABLE 11 Tension upon drawing T: 2.5 (N/fiber), Roller radius R:0.08/(m), T/R: 31.3, Number of pivotal swings: 1.67 (S⁻¹) Young'smodulus Breaking strength of of coating (MPa) coating (MPa) 1^(st) layer2^(nd) layer Void Peeling Example 22 4.0 0.4 900 ∘ ∘ Example 23 2.0 0.4900 x ∘

When the PMD value of the coated optical fiber 24 was measured, themeasured value was 0.2 ps/km^(1/2) or less, thus being favorable. Themicrobending loss increase was 1 dB/km or less, and the frequency ofbreaking upon screening was {fraction (5/1000)} km or less, each being“good”.

EXAMPLE 23

A coated optical fiber 24 was obtained in the same manner as Example 22except that the amount of multifunctional monomer (nonanedioldiacrylate) in the composition of Rs2-0 shown in Table 8 was adjusted soas to lower the breaking strength of the first UV-curable resin layer to2.0 MPa.

In thus obtained coated optical fiber 24, the cross-sectional area,Young's modulus, average linear expansion coefficient α_(a), linearshrinkage ratio, hardening shrinkage ratio, effective linear expansioncoefficient, and FI in each of the first and second UV-curable resinlayers were calculated. The results were the same as those in Example22.

Also, as in Examples 1 to 8, thus obtained coated optical fiber 24 wasstudied in terms of low-temperature transmission and microbending, andwas subjected to a high-tension screening test. Their results were thesame as those in Example 22.

Furthermore, thus obtained optical fiber 24 was evaluated in terms ofthe occurrence of voids and the peeling between the glass optical fiberand the first UV-curable resin layer at their interface. Table 11 showsthe results.

When the PMD value of the coated optical fiber 24 was measured, themeasured value was 0.2 ps/km^(1/2) or less, thus being favorable in thisExample as well. The microbending loss increase was 1 dB/km or less, andthe frequency of breaking upon screening was {fraction (5/1000)} km orless, each being “good”.

From the foregoing results of Examples 1 to 23 and Comparative Example1, the following were found. Namely, it was found that Examples 1 to 23yielded the total FI of 3 [N] or less, thus sufficiently being able toprevent transmission loss from increasing in a low temperatureenvironment, whereas Comparative Example 1 yielded the total FIexceeding 3 [N], thus failing to prevent transmission loss fromincreasing in a low temperature environment sufficiently.

It was also found that the microbending became better when the Young'smodulus of the first UV-curable resin layer at 23° C. was 0.7 MPa orless.

Further, when Examples 9, 10, and 15 were compared with Example 16, itwas found that, when the Young's modulus of the second UV-curable resinlayer at 23° C. was 150 to 1000 MPa while the Young's modulus of thethird UV-curable resin layer at 23° C. was greater than 1000 MPa but notmore than 1500 MPa, the microbending further improved, the frequency ofbreaking upon high-tension screening became lower, and breaking washarder to occur due to external damages.

Also, from Examples 1 to 6, it was found that, when the adhesive forcebetween the first UV-curable resin layer and the optical fiber was 50 to200 N/m, peeling could sufficiently be prevented from occurring after alow-temperature transmission characteristic test.

Further, it was found that, when the breaking strength of the firstUV-curable resin layer was 1.8 MPa or higher, voids could sufficientlybe kept from occurring in the UV-curable resin layer, wherebytransmission loss could sufficiently be restrained from increasing in alow temperature environment due to the occurrence of voids.

Furthermore, when Examples 22 and 23 were compared with each other, itwas found that voids were likely to occur within the coating if thebreaking strength of the first UV-curable resin layer was less than 4.0(MPa). Therefore, it was found that the breaking strength of the firstUV-curable resin layer was preferably 4.0 (MPa) or higher.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, the coated optical fiber of the presentinvention and the coated optical fiber ribbon and optical fiber unitusing the same can sufficiently prevent transmission loss fromincreasing at a low temperature, and can practically be used insubmarine or land optical communications in a low temperatureenvironment.

1. A coated optical fiber comprising a silica type glass optical fibercoated with n layers (n being an integer of 2 or greater) of UV-curableresin; wherein the sum of respective contraction stress indices FIdefined in said n layers of UV-curable resin by the followingexpression:FI [N]=(Young's modulus [MPa] of the UV-curable resin layer at −40°C.)×(cross-sectional area [mm²] of the UV-curable resinlayer)×(effective linear expansion coefficient [10⁻⁶/°C.]/10⁶)×(temperature difference 190[° C.]) is 3 [n] or less.
 2. Acoated optical fiber according to claim 1, wherein the first UV-curableresin layer adhered to said silica type glass optical fiber in said nlayers of UV-curable resin has a Young's modulus of 0.7 MPa or less at23° C.
 3. A coated optical fiber according to claim 1, wherein the firstUV-curable resin layer in said n layers of UV-curable resin and saidsilica type glass have an adhesive force of 50 to 200 N/m therebetween.4. A coated optical fiber according to claim 1, wherein the firstUV-curable resin layer in said n layers of UV-curable resin has abreaking strength of at least 1.8 MPa.
 5. A coated optical fiberaccording to claim 1, wherein n is 3, the second UV-curable resin layeramong the three layers of UV-curable resin having a Young's modulus of150 to 1000 MPa at 23° C., the third UV-curable resin layer having aYoung's modulus greater than 1000 MPa but not more than 1500 MPa at 23°C.
 6. A coated optical fiber according to claim 1, wherein the firstUV-curable resin layer in said n layers of UV-curable resin is obtainedby curing a resin composition containing: an oligomer having a molecularweight of at least 5000; a multifunctional monomer having a methylenegroup with a carbon number of 5 to 11; and a monomer having aheterocycle and/or a monomer having a multi-membered ring, saidmultifunctional monomer having a weight ratio of 0.02 to 0.04 withrespect to said oligomer; wherein said first UV-curable resin layer hasa breaking strength of at least 4.0 MPa.
 7. A coated optical fiberaccording to claim 1, wherein said silica type glass optical fiber is adispersion-shifted fiber having an effective core area of at least 60μm².
 8. A coated optical fiber according to claim 1, wherein said silicatype glass optical fiber is a negative dispersion fiber; and wherein theratio (S/D) between chromatic dispersion D and dispersion slope S insaid silica type glass optical fiber at a given wavelength within thewavelength range of 1.52 to 1.62 μm is 0.001 to 0.004 (1/nm).
 9. Acoated optical fiber according to claim 1, wherein said silica typeglass optical fiber is a negative dispersion fiber; and wherein theratio (S/D) between chromatic dispersion D and dispersion slope S insaid silica type glass optical fiber at a given wavelength within thewavelength range of 1.52 to 1.62 μm is 0.004 to 0.020 (1/nm).
 10. Acoated optical fiber according to claim 1, wherein said n layers ofUV-curable resin were coated with a coloring layer.
 11. A coated opticalfiber ribbon comprising a plurality of coated optical fibers accordingto one of claims 1 to
 10. 12. An optical fiber unit comprising: acentral tension member; and a plurality of coated optical fibersarranged about said central tension member; wherein said coated opticalfibers comprise the coated optical fiber according to one of claims 1 to10.